JPH06263520A - Production of oxide superconductor - Google Patents

Abstract

PURPOSE:To raise degree of orientation of crystal particles of a sintered, to attain increase in density and to stably obtain a superconductor of oxide having a high critical temperature. CONSTITUTION:A mixture comprising oxides of elements constituting a oxide superconductor such as Bi-Sr-Ca-Cu-O-based oxide superconductor or oxide- forming compounds is molded or the mixture is calcined and molded and the molding is burnt in an oxidizing atmosphere. The sintered compact is subjected to hot forging treatment under >=100kg/mm<2> pressure for >=1 hour in such a way that the crystal particles are subjected to plastic deformation.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing an oxide superconductor, and more particularly to a method for producing an oxide superconductor having high density and high orientation.

[0002]

2. Description of the Related Art In recent years, oxide superconductors have been discovered as a material having a higher critical temperature Tc (temperature at which resistance becomes zero) than metal superconductors that have been conventionally used as superconductors, and their practical application is expected. Has been done.

At present, the oxide superconductors are mainly Y-Ba-Cu-O-based (hereinafter referred to as Y-based) and Bi.
-Sr-Ca-Cu-O system (hereinafter referred to as Bi system) 2
The species is known, and in the latter oxide superconductor, T
Three types are known: a high Tc phase with a c of 110K, a low Tc phase with a Tc of 80K, and a low Tc phase with a Tc of 20K or less. Is being promoted especially.

In order to put these oxide superconductors into practical use, it is required that they have a high critical temperature and a large critical current density (current value at zero resistance).

Since the crystals of the Bi-based oxide superconductor are composed of scaly particles, it is considered that the critical current density can be increased by orienting the scaly particles in one direction. There is. In addition, it is said that the fact that the relative density of the sintered body is increased and the sintered body is highly densified is a major factor in terms of characteristics.

Therefore, as a method for producing a high-density oxide superconductor, a hot press method of heating while applying a high mechanical pressure is adopted.

[0007]

[Problems to be Solved by the Invention] However, Bi
When a low-Tc phase calcined powder is hot-press fired in the production of a system oxide superconductor, densification itself proceeds, but high Tc and high Jc cannot be expected due to the small generation of high Tc phase. . Therefore, it is also proposed to further heat-treat the sintered body after the hot pressing to generate a high Tc phase, but the density tends to decrease conversely because grain growth occurs due to the heat treatment.

Further, the above method has some effect on the densification of the sintered body, but it is insufficient from the viewpoint of grain orientation, so that the Jc value of the obtained sintered body is at most 1.
The current value is 000 A / cm ² or less, which has not reached the practical level.

[0009]

[Means for Solving the Problems] In order to solve the above problems, the inventors of the present invention first calcined a calcined powder of a low Tc phase at atmospheric pressure to form a sufficiently high Tc phase, and then calcined the same. By performing so-called hot forging treatment in which heat treatment is performed while applying pressure to the bonded body, the Jc value is 1500 with high orientation and high density.
It was proposed that an excellent oxide superconductor of about 4500 A / cm ² can be obtained. It is possible to further increase the density and the degree of orientation of the sintered body by repeating this hot fogging treatment, but when a method for obtaining a higher Jc value was examined, the crystal grains in this hot forging treatment were examined. It was found that when the atoms inside are diffused in a direction perpendicular to the pressing direction and plastically deformed to form particles having a large aspect ratio on the pressing surface, an oxide superconductor having a high Jc value is obtained.

That is, according to the present invention, a mixture comprising an oxide of an element constituting an oxide superconductor or an oxide-forming compound is molded, or the mixture is calcined and then molded to obtain a molded body. After being fired once in an oxidizing atmosphere, the sintered body is heated under pressure at a pressure of 100 kg / mm ² or more for 1 hour or more to plastically deform the crystal grains.

The present invention will be described below with reference to the drawings.

Steps (a) to (c) in the production method of the present invention
Will be explained individually.

Formulation and molding step (a) The oxide superconductor can be formed by using an oxide powder of a metal element constituting the oxide superconductor or a carbonate or nitrate powder capable of forming an oxide by firing. Weigh and mix to proportions. Specifically, in the case of forming a high Tc phase in the Bi-based oxide superconductor described above, Bi ₂ O ₃ , Sr
When each powder of O, CaCO ₃ , and CuO is used and Sr is set to 2 in the atomic ratio, Bi is 1.8 to 2.
2. Weigh so that Ca is in the range of 2.0 to 3.5 and Cu is in the range of 3.0 to 4.5. Further, PbO was added to the above mixture for the purpose of increasing the production amount of the high Tc phase.
Powder, and K ₂ CO ₃ , Na ₂ CO ₃ , Li ₂ CO ₂
, Sr is 2, Pb is 0. ~ 0.5, K, Li, N
One kind selected from a can be mixed at a ratio of 0.05 to 0.6.

The mixed powder thus obtained is molded by a known molding means. If desired, the above mixed powder may be mixed in an oxidizing atmosphere at 700 to 850 ° C.
After calcination for about 20 hours, it is crushed and similarly shaped. According to this calcining step, when the mixed powder having the above-described composition is calcined, the oxide superconducting powder mainly composed of the low Tc phase is generated. As the molding method, press molding, extrusion molding, doctor blade molding and the like are adopted.

Baking step (b) Next, the molded body obtained as described above is heated to 840-85.
Baking is performed in an oxidizing atmosphere at 5 ° C. for about 5 to 200 hours.
By this firing, scaly crystals of low Tc phase are once produced, but as the firing proceeds, the low Tc phase is converted to the high Tc phase. If this firing is performed without applying pressure, a low-density sintered body is formed due to the growth of flaky crystals, and thus hot press firing may be performed. At the end of such a firing step, the scaly crystals of the sintered body are in a substantially non-oriented state.

Hot Forging Treatment Step (c) Next, the sintered body obtained in the step (b) is subjected to hot forging treatment. In this hot forging treatment, as shown in FIG. 1, the sintered body 1 is pressed by the press punches 2 and 3 in the direction A.
Appropriate heating means (not shown) while applying pressure to the
Heat with. It differs from the hot press method in that the method perpendicular to the pressing direction A for the sintered body 1 is in an open state.

By performing the hot forging treatment at this time, the characteristics after the step (b) can be improved by performing the treatment. According to the present invention, however, the crystal grains are used under the conditions during the heat and pressure treatment. The characteristics can be further improved by processing under the condition of causing plastic deformation.
Specifically, at a pressure of 100 kg / cm ² or more, 800 to
It is kept at a temperature of 850 ° C. in an oxidizing atmosphere such as the air for 1 hour or more, particularly 10 to 300 hours, but the plastic deformation of the crystal particles may be carried out for a long time by increasing the pressure when the temperature is low. When the pressure is low, it is necessary to set the temperature high and hold it for a long time. The hot forging treatment can be repeated to further increase the amount of plastic deformation.

Further, in the hot forging process, in FIG. 1, pressure is applied between the sintered body 1 and the press punches 2 and 3 via a ductile metal plate such as silver, gold or copper to further improve the orientation. Can be increased.

[0019]

According to the constitution of the present invention, in the step (c),
It is most important to heat and press the sintered body obtained in the firing step (b) to such an extent that the crystal grains are plastically deformed. According to this hot forging treatment step (c), the flake-like crystal particles produced in the firing step (b) are compressed, the densification progresses as a whole, and the flake-like particles are oriented, The contact area between particles is increased, and the critical current density can be increased. At this time, in the hot forging treatment at low temperature or low pressure for a short time, the slip of particles becomes dominant and the crystal particles are oriented,
At the same time, since the particles are destroyed, the degree of orientation progresses only to some extent.

However, for example, when it is treated at a high temperature and a high pressure for a long time, the once broken particles are plastically deformed by the diffusion of atoms into particles having a large aspect ratio on the pressing surface, which are deformed and activated. The degree of orientation is further improved. As a result, the critical current density of the oxide superconductor can be dramatically increased.

[0021]

The present invention will be described below with reference to the following examples.

Example 1 (I) As raw material powder, Bi ₂ O ₃ , PbO, SrC
The powder of O ₃ , CaCO ₃ , and CuO has a molar ratio of each metal of Bi: Pb: Sr: Ca: Cu = 1.93: 0.36:
After weighing so as to be 2: 3.17: 4.25,
It was calcined at 810 ° C. for 20 hours and pulverized to obtain a calcined powder containing a large amount of low Tc phase having an average particle size of 5 μm.

(II) This calcined powder was molded at a molding pressure of 1 ton / cm ² using a die of φ12 mm to have a thickness of about 1 mm.
A disk-shaped molded body of was obtained.

(III) When the molded body was fired in the atmosphere at a temperature of 840 ° C. for 150 hours, a sintered body having a specific gravity of 2.0 (based on the Archimedes method) was obtained. Further, when the structure was observed, scaly crystals of high Tc phase were randomly arranged.

(IV) Next, according to FIG. 1, this sintered body was subjected to hot forging treatment on the upper and lower surfaces of the sintered body under the conditions shown in Table 1.

When the specific gravity of the finally obtained sintered body was examined by the Archimedes method, X-ray diffraction was measured, and X was measured.
Based on the chart data of line diffraction, the following formula 1

[0027]

[Equation 1]

From the above, the orientation degree f of the (001) plane was obtained.

Further, regarding the above-mentioned sintered body, the current value when the current of the sample was gradually increased in liquid nitrogen based on the resistance method and a voltage of 1 μV / cm was generated at the high voltage terminal was obtained as the critical current density Jc. At the same time, the critical temperature Tc was also measured. Moreover, the presence or absence of plastic deformation was confirmed by observing the change in crystal grain shape before and after the treatment. The results are shown in Table 1.

[0030]

[Table 1]

As is clear from Table 1, by setting the hot forging treatment conditions so that the crystal grains plastically deform, it was possible to enhance the orientation and improve the critical current density and the critical temperature.

It is considered that in Sample No. 8 having an extremely long hot fogging treatment time, the compositional deviation was remarkable due to the evaporation of the volatile components (bismuth and lead), and the superconducting characteristics were deteriorated.

[0033]

As described above in detail, according to the method of the present invention, the degree of orientation of the crystal grains of the sintered body is increased by performing the hot forging treatment so that the plastic deformation of the crystal grains occurs. It is possible to achieve high density and to stably obtain an oxide superconductor having a high critical current density.

[Brief description of drawings]

FIG. 1 is a diagram for explaining a hot forging treatment step in the manufacturing method of the present invention.

[Explanation of symbols]

 1 ... Oxide superconducting sintered body 2,3 ... Press punch

Claims (1)

[Claims] 1. The following steps (a) to (c), (a) forming a mixture of oxides or oxide-forming compounds of the elements constituting the oxide superconductor, or temporarily preparing the mixture. A step of forming after firing, (b) a step of firing the molded body in an oxidizing atmosphere, (c) a pressure of 100 kg / m of the sintered body
A method for producing an oxide superconductor, comprising the step of subjecting the crystal grains to plastic deformation by applying a pressure and heat treatment at m ² or more for 1 hour or more.